This invention is directed generally to noise filters, and particularly to digital filters for reducing noise in television signals.
It is well known in the art that so-called "recursive" filters are well suited for removing noise from repetitive signals such as television signals whose information content does not ordinarily vary to a great extent from frame to frame. One proposal for such a noise filter is disclosed in U.S. Pat. No. 4,064,530.
In the above-mentioned patent, a summer is employed to receive and sum an incoming digitized television signal and an earlier received television signal which has been delayed for an interval corresponding to one television frame period. The output of the summer provides a noise reduced television signal and also provides the input to a one frame delay device, the output of the latter providing one input to the summer.
In order to reduce smearing which can result from such recursive filtering when motion is present in the television signal, the filter described in the above-mentioned patent employs two attenuators, one between the incoming television signal and its input to the summer, and another between the output of the one frame delay device and its input to the summer. The two attenuators are adjusted simultaneously as a function of detected motion in the television signal and such that their combined outputs are equal to unity.
The above-described filter suffers from the fact that a rather expensive system is required to include two attentuators. In addition, a mis-adjustment to either of the attenuators may result in noticeable distortion in the output of the filter. Hence, prior recursive filters for use with television signals and the like have not been entirely satisfactory.
Another problem associated with filters used to reduce noise in NTSC television signals has to do with the fact that the chroma component of the television signal undergoes a 180.degree. phase reversal for each television line. Because there are an odd number of television lines in one television frame, the summation of an incoming television signal with a frame-delayed television signal can result in cancellation of the chroma component in the summed signal. To avoid such cancellation, it has been proposed to invert only the chroma component of the frame-delayed television signal prior to summing it with the incoming signal. While such chroma inversion tends to solve the problem of chroma cancellation, prior attempts to achieve such inversion, particularly utilizing a digitized television signal, have not been entirely satisfactory.